Optical scanner devices, such as flat bed scanners, are well-known in the art and produce machine-readable image data signals that are representative of a scanned object, such as a photograph or a page of printed text. In a typical scanner application, the image data signals produced by a scanner may be used by a personal computer to reproduce an image of the scanned object on a suitable display device, such as a CRT or a printer.
A typical flat bed scanner may include illumination and optical systems to accomplish scanning of the object. The illumination system illuminates a portion of the object (commonly referred to as a "scan region"), whereas the optical system collects light reflected by the illuminated scan region and focuses a small area of the illuminated scan region (commonly referred to as a "scan line") onto the surface of a photosensitive detector positioned within the scanner. Image data representative of the entire object then may be obtained by sweeping the scan line across the entire object, usually by moving the imaging assembly, i.e., the illumination, optical and sensor devices with respect to the object. In some scanners only a portion of the imaging assembly moves, e.g., a document support platen or a mirror assembly. In other scanners the entire imaging assembly moves.
By way of example, the illumination system may include a light source (e.g., a fluorescent or incandescent lamp or an array of light emitting diodes (LEDs)). The optical system may include a lens and/or mirror assembly to focus the image of the illuminated scan line onto the surface of the detector.
The photosensitive detector used to detect the image light focused thereon by the optical system may be a charge-coupled device (CCD), or other photosensor devices such as a contact image sensor (CIS). A typical CCD may comprise an array of individual cells or "pixels," each of which collects or builds-up an electrical charge in response to exposure to light. The quantity of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, thus a CCD may be used to generate electronic data representative of the brightness or darkness of the image portion focused on each pixel.
Flat bed scanners and various components thereof are disclosed in U.S. Pat. No. 4,926,041 for "Optical Scanner" of David Wayne Boyd; U.S. Pat. No. 4,709,144 for "Color Imager Utilizing Novel Trichromatic Beam Splitter And Photosensor" of Kent J. Vincent; U.S. Pat. No. 4,870,268 for "Color Combiner And Separator And Implementations" of Kent J. Vincent and Hans D. Neuman; U.S. Pat. No. 5,038,028 for "Optical Scanner Aperture And Light Source Assembly" of Boyd, et al.; and U.S. Pat. No. 5,227,620 for "Apparatus For Assembling Components of Color Optical Scanners" of Elder, et al., each of which is incorporated herein by reference for all that is disclosed therein.
In most optical scanner applications, each of the individual pixels in the CCD are arranged end-to-end in a straight line, thus forming a linear array. At any given point of time during scanning each pixel in the CCD array corresponds to a related pixel portion of the illuminated scan line. The individual pixels in the linear photosensor array are generally aligned in the "cross" direction, i.e., perpendicular to the direction of movement of the illuminated scan line across the object (also known as the "scan direction"). Each pixel of the linear photosensor array thus has a length measured in the cross direction and a width measured in the scan direction. In most CCD arrays, the lengths and widths of the pixels are equal, typically being about a microns or so in each dimension.
The resolution of the sensor in the cross direction is a function of the number of individual cells in the CCD. For example, a commonly used CCD photosensor array in a low cost scanner contains a sufficient number of individual cells or pixels to allow a resolution in the cross direction of about 300 pixels, or dots, per inch (ppi or dpi), which is referred to herein as the "native resolution" in the cross direction.
The resolution in the scan direction is inversely related to the product of the scan line sweep rate and the CCD exposure time (i.e., the sampling interval). Therefore, the resolution in the scan direction may be increased by decreasing the scan line sweep rate, the CCD exposure time, or both. Conversely, the resolution in the scan direction may be decreased by increasing the scan line sweep rate, the CCD exposure time, or both. The "minimum resolution in the scan direction" for a given exposure time is that resolution achieved with scanning at the maximum scan line sweep rate. For example, a maximum scan line sweep rate of about 3.33 inches per second and a maximum exposure time of about 5 milliseconds will result in a minimum resolution in the scan direction of about 60 dpi.
The resolution in the cross direction may be increased over the native resolution in the cross direction by using various data interpolation techniques to increase the effective resolution in the cross direction. For example, some data interpolation techniques can be used to increase the effective resolution in the cross direction to 600 or even 1200 dpi with a CCD having a native resolution in the cross direction of only 300 dpi.
As mentioned above, the resolution in the scan direction is a function of the scan line sweep rate as well as the CCD exposure time. Therefore, the resolution in the scan direction can be varied by changing the scan line sweep rate, the CCD exposure time, or both. It should be noted that the resolution in the scan direction corresponding to a given maximum scan line sweep rate and CCD exposure time is fixed and represents the minimum resolution in the scan direction.
While the techniques described above are useful in increasing the resolution in both the cross and scan directions, they are not without their disadvantages. For example, the interpolation techniques used to increase the resolution in the cross direction may require substantial amounts of processor time and/or memory, requiring either increased time to perform the scan operation or requiring faster processors and/or more memory if such higher resolutions are to be achieved without adversely affecting the overall scan time. Similarly, increasing the resolution in the scan direction may require either slower sweep rates, CCD exposure times, or both. Decreasing the sweep rate increases the time required to perform the scan, whereas decreasing the CCD exposure time may result in decreased image quality, or may require a more sensitive CCD array if image quality is to be preserved at such decreased exposure times.